The present invention relates to a new and improved construction of a control system for driving and steering a driverless transport device.
In its more particular aspects, the present invention specifically relates to a new and improved construction of a control system for driving and steering a driverless transport device which contains at least two drive wheels as well as additionally a predetermined number of free-wheeling support wheels. Each one of the drive wheels is provided with a vertical steering axle and is pivotable through predetermined steering angles by means of the steering axle. A predetermined one of the at least two drive wheels is freely controllable with respect to its drive speed.
Driving and steering control systems of the aforementioned type can be generally used for driving and steering driveable objects like, for example, during the guided or dead reckoning travel of trackless vehicles in automatic floor-level conveyor installations.
There are known driving and steering control systems for driverless transport devices in which individually driven drive wheels are arranged for pivoting about a vertical axis. Thus, there has been described by the Assignee of the present invention, for example, in German Pat. No. 2,555,365 a steering control system for remotely controlled transport devices each of which comprises two drive wheels R.sub.A and R.sub.B and, if desired, additional free-wheeling supporting wheels. The drive wheels R.sub.A and R.sub.B are respectively driven at drive speeds V.sub.A and V.sub.B. Each of the drive wheels is steered by means of a guiding or directing line which determines respective drive wheel steering angles .alpha. and .beta.. The steering angles .alpha. and .beta. designate the angles which are formed between the running or travel directions of the respective drive wheels R.sub.A and R.sub.B and an interconnecting line extending through the steering axles of the drive wheels. For this purpose an antenna is associated and pivotable with each drive wheel. The antenna scans an electric guiding or directing cable and thereby pivots the associated drive wheel by means of a steering motor in such a manner that the antenna is continuously located above the guiding or directing track and guides the drive wheel along the guiding or directing track.
The drive speed V.sub.A of a predetermined drive wheel R.sub.A then is freely controllable via the guiding or directing cable and the antenna by means of a master control unit, while the drive speed V.sub.B of the other drive wheel R.sub.B is controlled in accordance with the relationship EQU V.sub.B =(cos .alpha./cos .beta.).V.sub.A (I)
One substantial disadvantage of a drive system designed in such a manner results from the circumstance that in transport devices of the aforementioned type the economically acceptable expense for the driving and steering control is limited. Therefore, the drive speeds generally cannot be adjusted to each other in relation to the associated steering angles with the required precision. In order to satisfy the relation (I) which represents a law of nature, therefore, electrical and mechanical balancing mechanisms positively develop in the system and such electrical and mechanical balancing mechanisms, particularly at high speeds and in extreme driving positions, disadvantageously affect the operation and maintenance of such transport devices. In the presently described control system the two wheel drives, in fact, operate as parts or members of independent speed regulators. Upon the occurrence of differences between the speeds projected onto the longitudinal axis of the transport device, the total drive torque is non-uniformly distributed over the two wheel drives in correspondence with the elasticity or flexibility of the individual regulating characteristics. In an extreme case this may result in the two wheel drives counteracting each other, i.e. that the transport device is driven by only one motor while the other motor operates in the manner of a generator and brakes the transport device. Evidently there results therefrom an unnecessarily increased current consumption which is considered particularly disadvantageous because of the battery-powered current supply.
Furthermore, the different torques result in additional structural stresses and unequal wear of the drive motors, transmissions and wheel tires or rims. The track guiding precision is detrimentally affected thereby and there is also impaired the maneuverability, particularly during controlled travel like, for example, with the assistance of a stored table for controlling the travel. As a consequence, such transport devices are rendered more difficult to operate and are considerably more expensive to maintain.
Further substantial disadvantages result from the circumstance that in the previously suggested transport system the maximum permitted steering angles .alpha. and .beta. between the running directions of the drive wheels and the interconnecting line through their vertical steering axles, must be smaller than +90.degree. and -90.degree.. The mechanically available pivoting range of the drive wheels extends from -90.degree. to +90.degree. and thus cannot be fully utilized for the reasons of the control technique. It is of particular disadvantage in the known control system that the steering angles of +90.degree. and -90.degree. are not available, so that interesting driving or traveling modes like, for example, "turning on location" must be dispensed with. The mobility and maneuverability is thereby restricted and additionally the guiding or directing tracks cannot be laid in a freely selectable manner.